Generally, aluminum alloys are protected by forming thereon an intermediate corrosion resistant conversion coating and then painting over the corrosion resistant coating. Therefore, the corrosion resistant coating must be intimately bonded to the aluminum surface and also provide the required adhesion with the desired final aluminum coating-i.e., paint.
One of the widely used processes for protecting aluminum alloys with a corrosion resistant intermediate coating is to coat the surface of the aluminum alloys with a protective conversion coating of an acid based hexavalent chromium composition or other heavy metal such as zirconium and titanium.
All of these processes leave a very thin film of a heavy metal salt on the aluminum alloy which prevents the metal from becoming dark when subjected to boiling water for periods of time up to 30 minutes. These coatings also provide a suitable base for the application and retention of other coatings, such as paint, to the aluminum alloy surfaces.
The hexavalent chromium provides a corrosion resistant coating which can withstand a salt fog bath for more than 168 hours. The coated aluminum alloy is placed in a salt fog at 95.degree. F. according to ASTM method B-117 for at least 168 hours and then removed. This requirement is necessary for may application. Further, the hexavalent chromium composition provides an intermediate coating which is receptive to the application and retention of other coatings, such as paints, to the aluminum alloy surfaces.
The excellent features of the hexavalent chromium composition, have made these compositions used extensively for the corrosion resistant protection of aluminum alloys and as an intermediate corrosion resistant coating.
However, the hexavalent chromium compositions have a serious side effect. Chromium is highly toxic and the spent chromium compositions provide an ecological problem. Many people in the industry are attempting to eliminate this ecologically damaging waste problem and it is very costly.
Corrosion resistant permanganate compositions have been suggested, as noted in our U.S. Pat. Nos. 4,711,667 and 4,755,224.